While the demand for monitoring cameras has been increasing in order to operate unmanned monitoring facilities, there has also been a demand in recent years for zoom lenses that operate both in the visible light region for operating with visible light during the day and in the near-infrared region for operating with near-infrared light at night. An example of such a zoom optical system including a zoom lens is described in Japanese Laid-Open Patent Application 2002-196235.
The zoom lens described in Japanese Laid-Open Patent Application 2002-196235 has a compact two-group construction with, in order from the object side, a first lens group having negative refractive power and a second lens group having positive refractive power. Such a construction with a lens group having negative refractive power on the object side has the advantage of being useful for providing a wide maximum field angle and for easily providing an appropriate rear focus. However, when a two-group lens construction of negative and positive lens groups from the object side is provided, the entire lens system becomes asymmetric, variations in aberrations accompanying zooming increase, and especially correction for chromatic aberration becomes important in order to achieve a lens system that operates both in the visible region and in the near-infrared region while obtaining good optical performance over the entire zoom range.
The zoom lens described in Japanese Laid-Open Patent Application 2002-196235 corrects aberrations well by satisfying specified conditions and by making the seventh and eighth lens elements from the object side, which belong to the image-side lens group, have negative and positive refractive powers, respectively, be made of materials having a large difference in Abbe numbers, and be joined together to form a lens component having negative refractive power.
Especially demanded as a zoom optical system for monitoring cameras is having a relatively large aperture, that is, small f-number, so that a subject can be imaged with adequate brightness for identification even in a low illumination environment. Also, in order to make it possible to monitor over a wide area, increasing the field angle at the wide-angle end is required. Furthermore, being compact and having a good optical performance are also becoming necessary. When an attempt to increase the brightness of the lens system is made, aberration correction becomes more difficult, and thus the difficulty of providing good optical performance over the entire zoom range also increases.
Due to recent progress in the technology of forming aspheric lenses, the conventional design limitation that lenses with aspheric surfaces should be formed with relatively small outer diameters and relatively small center thicknesses has become unnecessary, and even relatively large aspheric lenses can be formed. Given such a technological background, the present range of choices for lenses with aspheric surfaces is expanding.
As described above, monitoring camera lens systems should have chromatic aberrations favorably corrected in both the visible light region and the near-infrared region. Other requirements for monitoring camera lens systems include providing a bright image even in low light illumination by reason of the lens having a low f-number, and being compact. It is desirable to develop a lens system that satisfies these requirements.